Exhaust gases discharged from internal combustion engines include harmful or toxic substances such as NOx. It is known that an NOx storage reduction catalyst (hereinafter referred to as an NOx catalyst) is provided to decrease an amount of these toxic substances to be discharged. In this technique, as the purification capacity or performance of the NOx catalyst decreases in accordance with an increasing amount of NOx stored therein, a reducing agent is generally supplied to the NOx catalyst, so that the NOx stored in the NOx catalyst can be thereby reduced and released therefrom (hereinafter referred to as “NOx reduction process”).
Further, in order to eliminate SOx poisoning in which SOx in an exhaust gas is stored into the NOx catalyst to lower the purification performance thereof, the bed temperature of the NOx catalyst may sometimes be made higher, and at the same time a reducing agent may be supplied to the NOx catalyst (hereinafter referred to as “SOx poisoning recovery process”). In this SOx poisoning recovery process, the reducing agent is also used for the purpose of raising the bed temperature of the NOx catalyst.
In addition, the exhaust gases of the internal combustion engines also include particulate matter (PM) that contains carbon as a major component. In order to prevent the particulate matter from being released to the ambient atmosphere, there has been known a technique in which a particulate filter (hereinafter referred to simply as a “filter”) for trapping particulate matter is provided on an exhaust system of an internal combustion engine.
In such a filter, as an amount of accumulation of the particulate matter trapped increases, back pressure in the exhaust gas is raised due to a clogging of the filter, resulting in decreased engine performance. Thus, in order to prevent this, the temperature of the filter is often made higher, whereby the particulate matter trapped in the filter can be oxidized and removed (hereinafter referred to as “PM regeneration process”). In this case, too, fuel, which serves as a reducing agent, may sometimes be supplied to the filter so as to increase the temperature thereof.
As a method for supplying a reducing agent to an exhaust gas purification apparatus as stated above, there has been well known one in which a reducing agent addition valve is arranged on an exhaust passage at a location upstream of the exhaust gas purification apparatus, so that a reducing agent such as for example liquid fuel can be added to an exhaust gas in the exhaust passage by means of the reducing agent addition valve. Here, note that if the reducing agent addition valve is arranged immediately upstream of the exhaust gas purification apparatus, there will be a fear that the liquid fuel added from the reducing agent addition valve to the exhaust gas might flow into the exhaust gas purification apparatus before the liquid fuel has dispersed in an appropriate manner. As a result, the reducing agent might not sometimes be supplied to the entire exhaust gas purification apparatus in a uniform manner.
In relation to this, Japanese patent application laid-open No. 2004-308549 discloses a technique in which a part of exhaust gas is subjected to pressure application to generate a pressurized exhaust gas, which is supplied to the exhaust gas together with fuel added by a fuel addition unit. In this technique, the fuel is atomized or vaporized by controlling the timing at which the pressurized exhaust gas is to be supplied and the timing at which the fuel is to be added in synchronization with each other. In this technique, however, it is necessary to pressurize the exhaust gas by means of an air pump or the like, so the system construction of the fuel addition unit might become complicated, thus providing a hindrance to the reduction of costs.
In addition, Japanese patent application laid-open No. H5-240131 discloses a technique in which fuel being injected from a fuel injection valve into an intake pipe is atomized by the use of a stream of air supplied by an air pump. In this technique, the fuel is injected from the fuel injection valve in such a manner as to synchronize with the pulsation of the assist air.
Also, Japanese patent application laid-open No. H11-159320 discloses an exhaust gas purification apparatus for an internal combustion engine which is provided with a baffle plate that is rotating while shielding a part of an inflow surface of an exhaust gas flowing into an NOx catalyst, and a reducing agent supplying unit that injects a reducing agent to a shielding portion of the baffle plate in succession in synchronization with the rotation of the baffle plate.
Moreover, Japanese patent application laid-open No. 2005-120938 discloses an exhaust gas purification apparatus for an internal combustion engine which is provided with a first exhaust path and a second exhaust path that are branched from an exhaust passage at a location downstream of a reducing agent supplying unit, and a change-over valve that is able to change over a path through which an exhaust gas flows into the first exhaust path or the second exhaust path.
In this technique, the timing at which a reducing agent is supplied by the reducing agent supply unit is synchronized with the timing at which the path through which the exhaust gas flows is changed over by the change-over valve.
In the NOx reduction process or the SOx poisoning recovery process, however, it is necessary to control the air fuel ratio of an exhaust gas flowing into an NOx catalyst to be a rich air fuel ratio. Accordingly, by supplying an exhaust gas containing a high concentration of a reducing agent to the NOx catalyst (i.e., by deepening or increasing a rich spike), a locally deep rich atmosphere is sometimes intended to be formed in the NOx catalyst, thereby making it possible to improve the reduction efficiency of NOx and/or SOx.
However, in the techniques disclosed in the above-mentioned publications, even if the degree of dispersion of a reducing agent added to an exhaust gas could be raised, it would be difficult to form a locally deep rich atmosphere in an NOx catalyst. Accordingly, a technique has been desired which is able to control the degree of dispersion of a reducing agent added to an exhaust gas flowing into an exhaust gas purification apparatus in accordance with the content of process (e.g., NOx reduction process, SOx poisoning recovery process, PM regeneration process, etc.) to be performed on the exhaust gas purification apparatus with a simple construction.